Analog (or solid state) switches are widely used in electronic circuits to control flow of current from an input terminal of the switch to an output terminal of the switch in dependence on a digital control signal. Such a switch may comprise, as its switching element, a field-effect transistor, e.g. a metal-oxide-semiconductor field-effect transistor (MOSFET) or a junction field-effect transistor (JFET), having its source connected to the input terminal, its drain connected to the output terminal and its gate connected to a driver circuit that receives the control signal. As shown in "Analog Switches and Their Applications", published by Siliconix incorporated, 1976, at page 2-3, the driver circuit, in its simplest form, may comprise a bipolar transistor or a FET.
In order to provide a high switching speed while minimizing bleeding of current from the analog signal path, a diode may be interposed between the collector of the driver circuit's bipolar transistor and the gate of the FET switching element. When the switch is closed, the diode conducts only the gate leakage current of the FET switching element, whereas when the switch is open it conducts only the reverse diode leakage current.
A switch topology that may be implemented with junction MESFET technology is described in W. E. Hall, "Design of a Radiation Hardened GaAs Band Gap Reference", Proceedings of 1984 GOMAC Conference, 1984, pages 443-447. This switch is shown in FIG. 1 of the accompanying drawings, and comprises a junction MESFET 22, which is the actual switch element, a source follower transistor 24, a current source 26 that supplies tail current for the transistor 24, a resistor 28 connected to the gate of the transistor 22, a level shift string 30 connected between the resistor 28 and the current source 26, and a transistor 32 that switches control current I.sub.ctrl in dependence on the control voltage .phi.. When the control voltage .phi. is low, the transistor 32 is non-conductive and the voltage on the gate of the transistor 22 follows that on the source of the transistor 24, with a level shift due to the level shift string 30. The level shift is set so that the gate of the transistor 22 follows the source of that transistor (via the source follower action of the transistor 24) in such a way as to maintain the transistor 22 in its conductive state, i.e. closed, without forward-biasing the gate-to-channel junction of the transistor 22 and thus perturbing the signal path with charge injected from the driver circuit. When the control voltage .phi. goes high, the transistor 32 turns on and the additional current drawn through the resistor 28 causes the potential at the gate of the transistor 22 to fall sufficiently to turn the transistor 22 off, thus opening the switch. (In this specification, a switch that is described as being closed or on is conductive, providing a current path from its input terminal to its output terminal, whereas one that is described as being open or off is non-conductive, preventing flow of current.)
The circuit shown in FIG. 1 is subject to certain disadvantages. In particular, the circuit is limited to unipolar operation, i.e. the input signal applied to the transistor 22 must be at a positive potential, since the sources of the transistors 26 and 32 are both grounded. Since the control current I.sub.ctrl is switched on and off to effect the switching action, the power supply current will exhibit substantial variations, which can result in significant power supply noise and perturbation of the analog signal path via secondary peripheral paths. The voltage variation at the gate of the transistor 22 can change the current through the level shift string 30 by a significant amount, resulting in a variation of like magnitude but opposite sign in the current flowing in the source follower transistor 24, and this variation in current through the transistor 24 will increase the time required for the voltage at the source of the transistor 24 to stabilize after the switch is closed. Finally, the fact that the current flowing in the source follower transistor 24 is subject to change implies that the voltage at its source will also change, and this change in voltage will couple through the gate-to-source capacitance of the transistor 24 and perturb the analog signal at the source of the transistor 22.